Lamellar-phase polymorphism in interdigitated bilayer assemblies

Bilayers composed of 1-octadecanoyl-2-decanoyl-sn-glycero-3-phosphocholine (C(18)C(10)PC) adopt a mixed-interdigitated gel-phase packing where the short chains of the C(18)C(10)PC molecules pack end-to-end while their long chains span the entire hydrocarbon width of the bilayer. Calorimetric cooling scans of freshly prepared hand-shaken bilayer suspensions of C(18)C(10)PC exhibit a single exothermic phase transition at 14.6 degrees C, whereas suspensions incubated at temperatures below 2 degrees C for several days exhibit an additional phase-transition exotherm at 17.9 degrees C. Calorimetric and electron microscopic evidence is presented that low-temperature incubation of C(18)C(10)PC bilayer suspensions composed of liposomes of heterogeneous size leads to the conversion of those liposomes in the suspension below about 0.2 microns in diameter into planar lamellar sheets. These lamellar sheets are the origin of the phase-transition exotherm at 17.9 degrees C, whereas the phase-transition exotherm at 14.6 degrees C arises from the liposomes in the suspension. We also show that phosphatidylcholine bilayer suspensions, induced to interdigitate by ethanol, exhibit a similar thermotropic behavior. The implication of these findings for the reversibility of interdigitated gel to liquid-crystalline phase transitions and the role of phospholipid molecular geometry in the formation of interdigitated bilayers are addressed.